1. Field of the Invention
The present invention relates to a mobile communication system using an Orthogonal Frequency Division Multiplexing (OFDM) scheme, which will be referred to as an OFDM mobile communication system, and more particularly to an apparatus and a method for acquiring synchronization in time-varying frequency selective fading channel conditions.
2. Description of the Related Art
In a 4th generation (4G) mobile communication system, which is the next generation communication system, research has been actively pursued to provide users with services having various qualities of service (QoS) at high transmission speed. Specifically, in the current 4G mobile communication system, research is being carried out to support a high speed service capable of ensuring mobility and QoS in a Broadband Wireless Access (BWA) communication system such as a wireless Local Area Network (LAN) communication system and a wireless Metropolitan Area Network (MAN) communication system.
In the 4G mobile communication system, an OFDM scheme has been actively researched as a scheme available for transmitting data through wire and wireless channels at a high speed. The OFDM scheme, which transmits data using multi-carriers, is a special type of a Multi-Carrier Modulation (MCM) scheme in which a serial input symbol sequence is converted into parallel symbol sequences and the parallel symbol sequences are modulated with a plurality of mutually orthogonal sub-carriers before being transmitted.
The 4G mobile communication system requires broadband spectrum resources in order to provide a wireless multimedia service of high quality at high speed. However, when the broadband spectrum resources are used, a fading influence on a wireless transmission channel may be serious due to a multi-path propagation, and a further influence due to frequency selective fading may occur in a transmission band. Accordingly, for a high speed wireless multimedia service, because an OFDM scheme tolerant to frequency selective fading has larger gain as compared with a Code Division Multiple Access (CDMA) scheme, the OFDM scheme has been widely applied to the 4G communication system.
Usually, a mobile communication system supports a mobile communication service by means of frames between a Base Station (BS) and a Mobile Subscriber Station (MSS). Accordingly, the BS and the MSS must acquire mutual synchronization for transmission/reception of frames. In order to achieve synchronization, the BS transmits synchronization signals to the MSS so that the MSS can recognize the start of a frame transmitted from the BS. Then, the MSS receives the synchronization signals transmitted from the BS, checks a frame timing of the BS, and demodulates frames received based on the checked frame timing. Usually, in the synchronization signals, a training sequence stipulated in advance by the BS and the MSS is used.
Hereinafter, operations of a transmitter and a receiver in the OFDM mobile communication system will be briefly described. For convenience of description, it is assumed that the transmitter of the OFDM mobile communication system is a BS and the receiver of the OFDM mobile communication system is a MSS.
In the BS, input data are modulated with sub-carriers through a scrambler, an encoder and an interleaver. The BS provides various variable data rates. Different coding rates, interleaving sizes and modulation schemes may be obtained according to the data rates. Typically, the encoder uses a coding rate of ½, ¾, etc., and the interleaver has sizes determined according to the Number of Coded Bits per OFDM Symbol (NCBOS) in order to prevent burst errors from occurring. The modulation scheme may use a Quadrature Phase Shift Keying (QPSK) scheme, an 8 PSK scheme; a 16 Quadrature Amplitude Modulation (QAM) scheme, a 64 QAM scheme, etc., according to the data rates.
A predetermined number of pilot sub-carriers are added to the signals modulated with the predetermined number of sub-carriers by the afore-described construction, and the signals pass through an Inverse Fast Fourier Transform (IFFT) unit to generate one OFDM symbol. Then, guard interval signals are inserted into the OFDM symbol in order to remove Inter-Symbol Interference (ISI) in multi-path channel conditions, and the OFDM symbol passes through a symbol waveform generator. Finally, the OFDM symbol is input to a Radio Frequency (RF) processor, and the RF processor processes the OFDM symbol and transmits the processed OFDM symbol to the air.
The guard interval signals are inserted when the OFDM symbol is transmitted in order to remove the ISI between an OFDM symbol transmitted at a previous OFDM symbol time and a current OFDM symbol being transmitted at a current OFDM symbol time. Further, the guard interval signals are inserted by one of a ‘cyclic prefix’ scheme, in which predetermined last samples of an OFDM symbol on a time domain are copied and inserted into an effective OFDM symbol.
The MSS corresponding to the BS as described above performs a process inverse to that performed by the BS, and a synchronization acquisition process is additionally performed. First, a process for acquiring synchronization by means of a training symbol or guard interval signals having been preset for a received OFDM symbol must be performed in advance. The process for acquiring the synchronization represents a process for estimating a frequency offset and a timing offset, i.e., a symbol timing offset, and the training symbol represents a symbol for transmitting a training sequence.
Then, a data symbol, excluding guard interval signals, passes through a Fast Fourier Transform (FFT) unit and is restored to the sub-carrier signals including the predetermined number of pilot sub-carriers. Further, in order to overcome a path delay on actual mobile communication channels, an equalizer estimates channel conditions for received channel signals and removes signal distortion on the actual mobile communication channels from the received channel signals. The data, for which the channel conditions have been estimated by passing through the equalizer, are converted into bit sequences, pass through a deinterleaver, and are then outputted as final data via a decoder and descrambler for error correction.
As described above, the BS transmits pilot sub-carrier signals to the MSS. The BS transmits data sub-carrier signals and simultaneously transmits the pilot sub-carrier signals to the MSS. The reason for transmitting the pilot sub-carrier signals is for synchronization acquisition, channel estimation, and BS identification. The pilot sub-carrier signals operate as the training symbol in order to allow the channel estimation to be performed between the transmitter and the receiver. The MSS can identify a BS including the MSS by means of the pilot sub-carrier signals. The position, to which the pilot sub-carrier signals are transmitted, has been stipulated in advance between the BS and the MSS.
In the OFDM mobile communication system as described above, the frequency offset and the timing offset are estimated using the training symbol or the guard interval signals. A scheme for estimating the frequency offset and the timing offset by means of the training symbol has a problem in that a loss of data transmission efficiency is unavoidable because the training symbol must be transmitted in order to estimate the frequency offset and the timing offset.
A scheme for estimating the frequency offset and the timing offset by means of the guard interval signals has a low frequency and timing offset estimation capability, as compared with the scheme for estimating the frequency offset and the timing offset by means of the training symbol. However, with use of the guard interval signals for estimation it is not needed to transmit the training symbol in order to estimate the frequency offset and the timing offset, so that frequency offset and timing offset can be estimated by a relatively simple operation without a loss of data transmission efficiency.
As described above, because the OFDM mobile communication system uses the broadband spectrum, the fading influence on the wireless transmission channel may be serious due to the multi-path propagation, and a further adverse influence due to the frequency selective fading may occur in the transmission band. While the OFDM scheme is tolerant to frequency selective fading as compared with the CDMA scheme, etc., but the time-varying frequency selective fading functions as a significant factor determining the total capacity of the OFDM mobile communication system.
However, the currently used scheme for estimating the frequency offset and the timing offset by means of the guard interval signals has a problem in that it is difficult to directly apply the scheme to the OFDM mobile communication system, including time-varying frequency selective fading channel conditions, because the scheme models received signals in Additive White Gaussian Noise (AWGN) or time-varying flat fading channel conditions.